1. Field of the Invention
The present invention relates to a measuring apparatus, an exposure apparatus, and a device fabrication method.
2. Description of the Related Art
A projection exposure apparatus has conventionally been employed to fabricate a micropatterned semiconductor device such as a semiconductor memory or logic circuit or a liquid crystal device by using photolithography. The projection exposure apparatus projects and transfers a pattern formed on a reticle (mask) onto a substrate such as a wafer by a projection optical system.
Along with advance in micropatterning and an increase in packing density of semiconductor devices, the projection optical system is required to project and transfer the pattern of a reticle onto a substrate by exposure with a higher resolution. A minimum line width (resolution) that the projection exposure apparatus can transfer is proportional to the wavelength of light for use in exposure (exposure light), and is inversely proportional to the numerical aperture (NA) of the projection optical system. According to this principle, the shorter the wavelength of the exposure light, and the higher the NA of the projection optical system, the better the resolution. In view of this, in recent years, a KrF excimer laser (wavelength: about 248 nm) or an ArF excimer laser (wavelength: about 193 nm) is used as the exposure light source. The practical application of immersion exposure which increases the NA of the projection optical system by filling the space between the projection optical system and the substrate with a liquid is also in progress. Furthermore, the projection exposure apparatus is required to expose a wider exposure region.
To meet these demands, an exposure apparatus of the step & scan scheme (scanner) is becoming the mainstream in place of an exposure apparatus of the step & repeat scheme (stepper). The stepper herein means an exposure apparatus which reduces an almost square exposure region and performs full-plate projection exposure of the exposure region on a substrate. The scanner herein means an exposure apparatus which uses an exposure region having a rectangular slit shape (exposure slit), and relatively scans the reticle and the substrate at high speed, thereby exposing a large area with high accuracy.
Before a predetermined position on the substrate reaches the exposure slit, the scanner measures the surface position of the substrate at the predetermined position by a surface position detection unit of the oblique incidence system, and performs correction to match the substrate surface with an optimal imaging position in exposing the predetermined position. To measure not only the height (focus) of the substrate surface position but also the tilt of the surface, a plurality of measurement points are set at the preceding stage and succeeding stage of the exposure slit along the longitudinal direction of the exposure slit (i.e., a direction perpendicular to the scanning direction). Details of the focus and tilt measurement techniques are proposed in Japanese Patent Laid-Open No. 6-260391 and U.S. Pat. No. 6,249,351. For example, Japanese Patent Laid-Open No. 6-260391 discloses a technique which uses an optical sensor. U.S. Pat. No. 6,249,351 discloses a technique which uses a capacitance sensor.
However, in recent years, along with the shortening of the exposure light and an increase in the NA of the projection optical system, the depth of focus is becoming very small and therefore the accuracy of matching the surface of a substrate to be exposed with an optimal imaging position, that is, the so-called focus accuracy is increasingly becoming stricter. More specifically, measurement errors attributed to the performance of an optical system which forms the surface position detection unit are becoming non-negligible in the surface position detection unit.
U.S. Pat. No. 6,249,351 discloses a technique of measuring the shape of a substrate based on an interference signal obtained by obliquely irradiating the substrate with light, as shown in FIG. 11. However, the technique disclosed in U.S. Pat. No. 6,249,351 measures the shape of a measurement target object corresponding to a wafer with an error due to the influence of the optical imaging performance of an optical system formed from lenses which are inserted between the measurement target object and an image sensing device, and adjust the measurement target object and the image sensing device to have an imaging relationship. Note that FIG. 11 is a schematic view showing the arrangement of a conventional measuring apparatus which measures the shape of a measurement target object.
FIG. 12 shows interference signals (the so-called white light interference signals) obtained by scanning the measurement target object in a direction perpendicular to its surface via an actuator in the conventional measuring apparatus shown in FIG. 11. The interference signal in case 1 shown in FIG. 12 is the one obtained under a relatively small influence of the aberration of the optical system (lens). On the other hand, because the optical system (lens) generally has wavelength-dependent aberration amounts (the so-called chromatic aberration) as shown in FIG. 13, an interference signal as in case 2 shown in FIG. 12 is obtained in the conventional measuring apparatus. The interference signal in case 2 is distorted due to the influence of the aberration of the optical system (lens), as compared with that in case 1. Such distortion of the interference signal generates an error in the measurement value of the substrate shape profile. It is possible to decrease the aberration of the optical system by increasing the number of lenses of the optical system or using an expensive glass material for the optical system. However, a broadband light source is desirably used to obtain a white light interference signal, so it is very difficult to decrease the aberration of the optical system. Even when an optical system having a small aberration can be attained by increasing the number of lenses of the optical system or using an expensive glass material for the optical system, this entails a significantly high cost.
As a commonly-used technique of improving the contrast of the white light interference signal, it is known to widen the wavelength range of light which irradiates the substrate. When the substrate surface is coated with a thin film such as a resist, it is thought to be effective to widen the wavelength range on the short-wavelength side. In this case, the transmittance on the short-wavelength side is often relatively low depending on the glass material used, which may exert an adverse influence on the interference signal.
FIG. 14 is a graph showing interference signals (white light interference signals) obtained by widening the wavelength range of light which irradiates the measurement target object and scanning the measurement target object in a direction perpendicular to its surface in the conventional measuring apparatus shown in FIG. 11. The interference signal in case 3 shown in FIG. 14 is the one obtained when the absorbance of the glass material on the short-wavelength side is low (i.e., the transmittance is high). The interference signal in case 4 shown in FIG. 14 is the one obtained when the absorbance of the glass material on the short-wavelength side is high (i.e., the transmittance is low). Referring to the interference signal in case 4, light components in the short-wavelength range are absorbed so that a desired low coherency may not be obtained, resulting in the generation of an error in the measurement value of the substrate shape profile.
In a phenomenon called solarization, a glass material absorbs light components having short wavelengths, it suffers an internal defect and therefore its transmittance on the short-wavelength side decreases. When this occurs, depending on the time taken to irradiate the measurement target object with light, the spectroscopic characteristics of a light component reflected by a reference surface and those of a light component reflected by the substrate surface change, resulting in the generation of an error in the measurement value of the substrate shape profile.